1. Field of the Invention
The present invention relates to a method of overlaying by a laser beam and an overlaying structure in which a laser beam is irradiated while putting the other metal material other than a base metal material to a base metal surface so as to melt the other metal material and form an overlay layer.
2. Description of the Related Arts
A technique for forming an overlay layer made of the other metal material other than a base metal on the base metal by irradiating a laser beam in accordance with the related art is applied to, for example, a valve seat portion in a cylinder head of an engine for an automotive vehicle (for example, refer to Japanese Patent Application Laid-Open No. 2-196117).
Since the valve seat portion mentioned above is under a serious condition such as a repeated contact by the valve during an engine operation, an exposure to a high temperature and the like, in the case of making the cylinder head mentioned above made of an aluminum alloy in view of lightening, there is a risk that an abrasion and a welding loss are generated, so that an overlay layer by the other metal material having a high heat resistance and abrasion resistance is formed.
In the technique for overlaying the other metal material, since a tightness between a cylinder head body (a base metal) and an overlay layer corresponding to a valve seat material is high in comparison with a conventional technique of pressing a valve seat material made of a cast iron and an iron sintered material to a recess portion formed in the valve seat portion, a heat conductivity between both materials is good, so that a thermal load in the valve seat portion and the valve is reduced and an improvement of a durability and an improvement of an output performance in an engine can be achieved.
In the case of forming the overlay layer on a ring-like portion such as the valve seat mentioned above, it is necessary to form an overlap portion by overlapping a terminal end portion with a beginning end portion corresponding to an overlaying operation starting portion for the overlay layer. Accordingly, an amount of the overlay can be secured in the portion.
The structure of the overlap portion is large at a degree of 70 degrees of an inclined angle .theta. with respect to a surface of the base metal on the surface of the beginning end portion. In such a case, at a time when the terminal end portion starts overlapping with the beginning end portion, since the inclined angle .theta. is large, the terminal end portion in a melting state is hard to be attached to the beginning end portion, and the overlap starting portion of the overlay layer with respect to the surface of the beginning end portion S on the base metal is hard to be melted with the base metal and the beginning end portion, so that the portion becomes a non-deposited portion and generates a crack. On the contrary, in the case of making the inclined angle .theta. small as a degree of about 5 degrees, the structure of the overlap portion is made such that a melting amount of the base metal is increased due to an excess thermal conduction to the base metal and a mixing amount of the base metal to the overlay layer in a melting state is increased, so that a crack is easily generated in the overlay layer after solidification.